Answer:
Collision between Indo-Australian plate and Pacific plate
Step-by-step explanation:
Due to the collision between the Indo-Australian plate and Pacific plate, the pacific plate being denser got subducted beneath the less denser Indo-Australian plate. This subducting plate undergoes partial melting, due to which the kermadec volcanoes are formed.
There are many volcanoes here out of which only four are found above the sea level, the remaining are present below the sea level forming sea mounts. Along this volcanic island arc, there are both active as well as inactive volcanoes present and due to the rapid and continuous erosion by the waves, some extinct volcanoes have turned into small islands.